Radio frequency identification (RFID) systems typically use one or more RFID reader antennae to send radio frequency (RF) signals to items tagged with RFID tags. The use of such RFID tags to identify an item or person is well known in the art. In response to the radio frequency (RF) signals from an RFID reader antenna, the RFID tags, when excited, produce a disturbance in the magnetic field (or electric field) that is detected by the RFID reader antenna. Typically, such tags are passive tags that are excited or resonate in response to the RF signal from an RFID reader antenna when the tags are within the detection range of the RFID reader antenna.
The detection range of the RFID systems is typically limited by signal strength to short ranges, for example, frequently less than about one foot for 13.56 MHz systems. Therefore, portable reader units may be moved past a group of tagged items in order to detect all the tagged items, particularly where the tagged items are stored in a space significantly greater than the detection range of a stationary or fixed single RFID reader antenna. Alternately, a large RFID reader antenna with sufficient power and range to detect a larger number of tagged items may be used. However, such an antenna may be unwieldy and may increase the range of radiated power beyond allowable limits. Furthermore, these RFID reader antennae are often located in stores or other locations where space is at a premium and it is expensive and inconvenient to use such large RFID reader antennae. In another possible solution, multiple small antennae may be used but such a configuration may be awkward to set us when space is at a premium and when wiring is preferred to be hidden.
Current RFID reader antennae are designed so that a sufficient read range maybe maintained between the antenna and associated tags, without running afoul of FCC limitations on radiated emissions.
Resonant RFID reader antenna systems are currently utilized in RFID applications, where numerous RFID reader antennae are connected to a single reader. Each RFID reader antenna may have its own tuning circuit that is used to match to the system's characteristic impedance. An RFID reader or interrogator maybe connected to more than one antenna, for example, to save cost or complexity of using many readers, particularly if a fast read rate is not necessary. However, sharing a reader means that a given antenna may not be read as often. It may be desired, however, to know immediately when an item is removed from the vicinity of an antenna, or at least to monitor the antenna for a time thereafter. It may therefore be useful to develop methods that allow the RFID reader to prioritize the order in which antennae are read.